Follow up https://github.com/nim-lang/Nim/pull/19512
ref https://github.com/nim-lang/Nim/issues/24794
Otherwise, `/Users/blue/Desktop/Nim/lib/system/mm/malloc.nim(4, 1)
Error: redefinition of 'allocImpl'; previous declaration here:
/Users/blue/Desktop/Nim/lib/system/memalloc.nim(51, 8)`
In `proc allocImpl*(size: Natural): pointer {.noconv, rtl, tags: [],
benign, raises: [].}`, `rtl` means it is an `importc` function instead
of a proc forward decl.
(cherry picked from commit d15705e05b)
e68a91c8df